Known are control units for controlling electric drive motors of industrial sewing machines, which comprise a control arrangement of the foot pedal type to operate a device which senses the foot pedal position and is connected to the electric motor. The foot pedal position-sensing device outputs a signal which varies consistently with the foot pedal position and drives the electric motor accordingly.
The foot pedal can be operated in two opposite directions. When operated in one direction, the foot pedal moves from a starting position where the electric motor is de-energized to an end position where the electric motor is energized at its maximum speed, the intermediate positions to the start and end positions of the foot pedal providing intermediate speeds of the electric motor. When operated in the opposite direction, the foot pedal moves to a first position where the cloth down-pressing shoe is raised and then to a successive position where the needle is raised.
The device for sensing the foot pedal position may either employ photocells or electric contacts to be closed sequentially by the pedal movement. Such prior arrangements, however, are cost-intensive, prone to fail, and have a low resolution.
To overcome such deficiencies, arrangements have been provided which utilize the Hall effect, wherein a permanent magnet is brought, by operation of the foot pedal, toward away from a Hall effect sensor according to the pedal movement; the Hall sensor, being suitably powered, will issue an electric voltage signal consistent with the position of said magnet. This output signal then drives the electric motor accordingly, via a control processor.
The currently available control units for the electric motors of industrial sewing machines have regrettably a serious drawback in that they disallow any proper interaction between the operator and the machine. That is, such control units provide no assistance, as expected instead of them, in translating the command from the operator into a corresponding operation of the electric motor driving the machine.
One of the causes for this resides indeed in the Hall effect detector. In fact, the pattern of the Hall effect sensor voltage output versus the magnet position is a hyperbolic one, and accordingly, when the magnet lies close to the sensor, a small movement of the foot pedal is enough to produce a large change in the sensor output voltage, whereas when the magnet is held away from the sensor, large movements of the pedal are required to bring about significant changes in that output voltage. This results in a non-linear response of the control unit which hinders control of the electric motor as desired by the operator. In addition, any small changes in the sensor temperature will reflect in minor variations of its output voltage which, with the magnet held away from the sensor, are erroneously taken as movements of the foot pedal.